I don't understand how we make the approximation that
1/2 [1 - 2δ + sqrt(1 + 4δ - 4δ^2)] \approx 1/2 [1 - 2δ + (1 + 2δ - 6δ^2)]
I see that we could for instance expand sqrt(1 + 4δ - 4δ^2) with Taylor series around 0, but that would lead to 1 + 2δ - 4δ^2 + ..., (notice -4δ^2 not -6δ^2).
Thank you for your help